Abstract Microfluidic-based techniques have been shown to address limitations of reconfigurable radio frequency (RF) antennas and filters in efficiency, power handling capability, cost, and frequency tuning. However, the current devices suffer from significant integration challenges associated with packaging, actuation, and control. Recent advances in reconfigurable microfluidics that utilize the motion of a selectively metalized plate (SMP) for RF tuning have demonstrated promising RF capabilities but have exposed a need for an accurate fluid actuation model. This research presents a model for the mechanical motion of a moving plate in a channel to relate the SMP size, microfluidic channel size, velocity, and inlet pressure. This model facilitates understanding of the actuation response of an RF tuning system based on a moving plate independent of the actuation method. This model is validated using a millimeter-scale plate driven by a gravitational pressure head as a quasi-static pressure source. Measurements of the prototyped device show excellent agreement with the analytical model; thus, the designer can utilize the presented model for designing and optimizing a microfluidic-based reconfigurable RF device and selecting actuation methods to meet desired outcomes. To examine model accuracy at device scale, recent papers in the microfluidics reconfigurable RF area have been studied, and excellent agreement between our proposed model and the literature data is observed.

Abstract The present paper investigates the rotating vortex rope (RVR) mitigation on an axial turbine model by the radial protrusion of four cylindrical rods into the draft tube. RVR mitigation is of particular interest due to the unfavorable pressure pulsations it induces in the hydraulic circuit that can affect turbine life and performance. The protrusion lengths, which were the same among the four rods, were varied according to a predefined sequence. The experiments were performed under four part-load regimes ranging from upper part load to deep part load. Time-resolved pressure measurements were conducted at two sections on the draft tube wall along with high-speed videography and efficiency measurement to investigate the effect of the mitigation technique on the RVR characteristics and turbine performance. The recorded pressure data were decomposed and studied through spectral analyses, phase-averaging, and statistical analyses of the RVR frequency and peak-to-peak pressure amplitude distributions. The results showed different levels of pressure amplitude mitigation ranging from approximately 10% to 85% depending on the operating condition, protrusion length, and the method of analysis. The hydraulic efficiency of the turbine decreased by a maximum of 3.5% that of the best efficiency point (BEP) with the implementation of the mitigation technique. The variations in the obtained mitigation levels and efficiencies depending on protrusion length and operating condition indicate the need for the implementation of a feedback-loop controller. Thus, the protrusion length can be actively optimized based on the desired mitigation target.

Abstract Cavity flows studied over the past few decades have led to an increased understanding of the flow physics and instability modes in a range of configurations. While a large number of these studies focus on two-dimensional and three-dimensional rectangular/cuboidal cavities, significant variations are seen with three-dimensional cylindrical cavities with a top bounding wall. The present work details the flow physics of such cavities with a compressible (air) flow past the cavity at two pressure drops of 3850 Pa and 2000 Pa. Results from detached eddy simulations (DES) reveal the presence of the wake mode and shear layer instabilities, respectively, with modified dynamics and oscillatory modes owing to the top wall. In the interest of preventing mode switching with changes in operating conditions, which could lead to large-scale flow disruptions, a passive flow control technique is tested. The modified cavity (with a downstream ramp scaled by the size of shed structures) is seen to maintain the same mode throughout the range of operation offering valuable insights into design modifications for such cavities in practical settings.

Abstract Centrifugal pumps with guide vanes (GVs) are widely used in a pump as turbine, energy storage pump station, and water diversion project. The objective of this study is to establish a design method of centrifugal pumps for matching impeller outlet and diffuser inlet to reduce hydraulic loss. To this end, a theoretical prediction model based on the fluid governing equation and the Oseen vortex model is proposed to predict the velocity moment downstream the impeller of the centrifugal pump. Then, a design method is established to design the impeller of the centrifugal pump with guide vanes. A centrifugal pump with a specific speed of 127 is used to validate the theoretical prediction model, and results of velocity moment show that the deviation between predicted and simulated results is below 0.5% in average. Finally, the design method is applied, and results show that the average efficiency of redesigned pump under the working conditions is 1.04% higher than that of baseline pump, which validates the reliability of proposed design method by theoretical prediction based on Oseen vortex. Analysis on velocity distribution and turbulence eddy dissipation shows that the design method based on Oseen vortex can effectively improve the flow pattern and pump performance.

The Reviewers of the Year Award is given to reviewers who have made an outstanding contribution to the journal in terms of the quantity, quality, and turnaround time of reviews completed during the past 12 months. The prize includes a Wall Plaque, 50 free downloads from the ASME Digital Collection, and a one year free subscription to the journal.S. AbdallahA. AbdehkakhaT. AcharyaA. AkturkE. AlawadhiS. AleyasinM. AliS. AliH. AlnawafahE. AlpmanT. AmiO. AmiliK. AndersonV. AndichamyL. AndolfattoC. AndreasenD. ArivoliD. AslangilW. AsrarA. AssamP. AtanasijevicS. aus der WiescheE. AvitalE. AyderH. BaiR. BalaguruA. BanerjeeL. BaranyiL. BarbacaS. BasirB. BatsonF. BattagliaE. BeniniL. BergS. BhushanM. BilgicY. BinG. BoisP. BrandnerS. CaiT. CarolusE. CasartelliP. ChangH. ChansonG. ChenH. ChenL. ChenP. ChenT. ChyczewskiJ. CimbalaJ. CoderL. ColomboA. CorsiniO. Coutier-DelgoshaJ. CuiY. CuiA. DaniD. DeS. DeivandrenG. DelibraS. DenizD. DukeH. EckstaedtB. ElbingM. EldeebT. ElGammalP. EppleT. EvansA. FalcaoB. FanD. FangX. FangA. FavrelZ. FengJ. Fernãndez OroC. FerrariM. FrancoisJ. FriedrichsM. FritscheG. FuT. FuJ. GaoM. GenãC. GentnerA. Gomes de-FreitasJ. Gonzalez PerezG. GutzeitK. HammadT. HeindelH. HoriguchiS. HoseinzadehX. HuangI. IlievM. JarrahiR. Jefferson-LovedayB. JiX. JiaW. JianP. JoshiK. KanL. KavurmaciogluG. KhokharM. KhozaeiD. KimJ. KimK. KimY. KimM. KinzelA. KnizleyS. KollaD. KozulovicD. KukulkaV. KulkarniR. KunzA. LeT. LeeM. LeiC. LiD. LiH. LiJ. LiP. LiX. LiZ. LiJ. LindauJ. LiouM. LiuZ. LiuO. Lopez MejiaA. LotfianA. LucchiniX. LuoD. MaJ. MajdalaniM. MalikX. MaoH. MaralL. MarghaL. MaroccoM. MasiC. MathpatiR. MeherF. MenterA. MishraC. MortonS. MousaviS. MunteanJ. MusserS. MuthuB. NaranjaniC. NegrãoA. NekkantiT. NewM. NohmiH. NowruziJ. OkajimaA. PandeyL. ParrasP. PelzM. PendarI. Perez-RayaS. PetersonM. PetkovsekG. PezzingaJ. PralitsM. PuccioniV. RadhakrishnaG. RajanW. ReflingZ. RenhuiW. RiderE. RobertsonG. SaccomandiA. SadagopanA. SagharichiO. SchillingD. SchmidtV. SchoepfM. ShahG. SharmaT. ShihH. ShimO. ShobayoQ. SiA. SinghA. SircarR. SkodaW. SongS. SridharS. SrivastavaT. StaubliW. StrasserX. SuS. Suhas PolP. SullivanD. SumnerD. SunX. SunM. TachieD. TanP. ThakurY. TsujimotoB. TunaN. UddinM. van der SchootT. van DruenenB. van EschG. VazJ. VelechovskyF. VisserD. WaltersB. WangP. WangY. WangH. WarriorH. WatanabeS. WatanabeJ. WatersB. WeiZ. WeiZ. XiaY. XiangC. XiaoH. XuJ. XuT. YamadaK. YamamotoY. YanF. YangX. YangR. YerramK. YonezawaJ. YuM. YuF. YuanM. ZangenehA. ZarandiD. ZhangJ. ZhangN. ZhangN. ZhangW. ZhangY. ZhangL. ZhaoB. ZhuZ. ZhuZ. Zou

Abstract Development and application of the open-source GPU-based fluid-thermal simulation code, NekRS, are described. Time advancement is based on an efficient kth-order accurate timesplit formulation coupled with scalable iterative solvers. Spatial discretization is based on the high-order spectral element method (SEM), which affords the use of fast, low-memory, matrix-free operator evaluation. Recent developments include support for nonconforming meshes using overset grids and for GPU-based Lagrangian particle tracking. Results of large-eddy simulations of atmospheric boundary layers for wind-energy applications as well as extensive nuclear energy applications are presented.

Abstract Thermoacoustic instability for chemically reacting flows was investigated using large eddy simulations coupled with a lookup table for turbulence-chemistry closure. The onset of instability was evaluated from pressure fluctuations, as well as standard and extended Rayleigh criterion, as suggested in literature. Two configurations were considered, namely, a canonical Rijke tube and a simplified can combustor with a swirling flow injector representing a complex generalized geometry. For the Rijke tube, premixed and nonpremixed combustion models were applied for identical fuel flowrate, resulting in different thermoacoustic outcomes due to differences in reaction rates of the two flame regimes. Results from the Rijke tube case agree with analytic thermoacoustic theory. For the can combustor, only premixed chemistry was considered as it better represents the experimental conditions, and the first resonant pressure mode aligns reasonably with published experimental data. Findings suggest that, if thermoacoustic instability is detected, the resonant frequency can be deduced from the fluctuations of the pressure, heat release, or acoustic source term. However, even though the resonant frequency is correctly identified, fluctuation data alone is insufficient to identify the onset of thermoacoustic instability, requiring the additional application of Rayleigh criterion. Finally, this study concludes that, for the range of configurations evaluated here, the standard Rayleigh criterion is sufficient to determine the onset of thermoacoustic instability, so the extended Rayleigh criterion is not always necessary, in contrast to suggestions from previous work. This conclusion is significant because the standard Rayleigh criterion is the only practical evaluation for physical experiments.

Abstract Traditionally fluidic oscillators are designed to be planar. However, there are applications that may desire the exiting fluid to move in the third dimension. This could allow these oscillators to be more effective in applications such as fuel sprays, cooling flow, or flow control devices with its increase in effective spray area. This investigation designed a series of oscillators that curved the whole body and/or the exit nozzle to understand how to maximize out of plane motion. These configurations were compared to a baseline planar oscillator with no curved characteristics. Velocities were measured downstream of these oscillators within a data collection grid using a hot wire probe to determine the 3D shape of the exiting jet. Results show that configurations with only one of the two curved physical characteristics (i.e., only a curved body or a curved nozzle) produced the most curvature. Having both of the curved physical characteristics caused the nozzle width to decrease causing the axial spacing to decrease. Additionally, these curved exiting flows were only seen at mass flow rates below 40 standard liters per minute (SLPM). Higher mass flow rates caused the exiting flow to flatten, returning the flow to the baseline result of in-plane oscillations. This led to a decrease in jet spread.

Abstract Studies have shown that the runner speed of hydraulic turbines at no-load conditions is affected by cavitation. However, those studies did not provide explanations relating the variation of the no-load runner speed to cavitation. Understanding why cavitation affects the runner speed is crucial because the maximum runner speed is reached in no-load condition, and this speed must remain below a limit to ensure the generator's safety. This paper uses numerical simulations to investigate the effect of cavitation on two no-load conditions, the runaway and the speed-no-load, for a low specific speed Francis turbine at model scale. The study is based on unsteady Reynolds-averaged Navier–Stokes simulations with and without cavitation and focuses on averaged quantities. At no-load, the regions over the blades producing a motor torque, i.e., oriented with the turbine rotating direction, must be balanced by regions producing a braking torque, opposed to the turbine rotation, to achieve a zero-torque condition. At runaway, cavitation mainly affects regions where a motor torque is produced. However, the zones affected by cavitation have a small contribution to the total motor torque. Therefore, for the runaway condition studied, the torque balance over the blade is hardly affected by cavitation, and the impact of cavitation on the runaway speed is negligible. At speed-no-load, comparisons between cavitating and noncavitating simulations indicated that cavitation affects mainly the braking torque regions. Those regions result from an interaction between the runner blades and a backflow extending from the draft tube cone to the runner outlet. In that case, cavitation strongly affects the torque balance over the blades, and consequently, the runner speed will adapt to find another zero torque condition.

Abstract The effects of perforation on the characteristics of the downstream vortex of a microvortex generator (MVG) mounted on a flat plate, are numerically investigated in this paper. A single rectangular MVG with circular and square perforation, at different locations on the MVG, is mounted on a flat plate at 16 deg installation angle with respect to the flow direction. The MVG is placed inside a turbulent boundary layer at Reynolds number of 16,293 based on MVG height and freestream velocity. The analysis is conducted using the Reynolds-averaged Navier–Stokes (RANS) modeling. The results of the analysis show that the MVG vortex is most effective when the perforation is located at the center and lower side of the MVG. The vortex strength for MVG with this perforated location is found to be much higher than the MVG without any perforation.